In a local area network (LAN) environment a user, such as a portable computer equipped with communication capability, gains access to the LAN via a physical connection in order to communicate with remote facilities or use shared resources, such as file servers, print servers, etc. In a stationary mode of operation, all users are static and each user gains access to the network via a fixed homing point. However, in a mobile environment users are free to change their physical location and cannot be restricted to gain access to the network only through one of several homing points attached to the LAN. In a mobile environment the homing points are fixed header stations that communicate with the mobile units through a wireless link. Examples of wireless links include radio frequency (RF) links, microwave links and infrared (IR) links.
For example, a wireless network may include two Header Stations (HS.sub.1 and HS.sub.2) attached to a wired network. Each header station has an associated wireless communication coverage area, or cell. All communication traffic originating from or destined for a first mobile data processing unit is managed by HS.sub.1, also considered to be the "owner" of the first mobile data processing unit. Similarly, all communication traffic originating from or destined for a second mobile data processing unit is managed by HS.sub.2, the owner of the second mobile data processing unit. As a user moves from one physical location to another, the header station responsible for managing the communication needs of the user must also change. Management of the communications needs of a mobile unit includes routing messages from the mobile unit to the LAN and broadcasting messages from the LAN to the mobile unit.
As a mobile unit moves from cell to cell, the mobile unit's owner is required to change. The type of mobile communication of interest herein implies that mobile units communicate with one another, via a header station or stations, using symbolic names and without any specific reference to a physical location at which the mobile unit may presently be positioned. As such, a number of suitable communication protocols are required to manage the change in ownership of a given mobile unit as it changes position relative to the fixed header stations. These protocols include the following.
(a) Establishment of unique ownership for each mobile unit that become active, or turned on, for the first time in the system.
(b) Detection of movement of mobile units as they cross from one communication cell to another.
(c) Accomplishing the change of ownership of mobile units as movement from one cell to another occurs.
(d) The assignment of a unique owner to a mobile unit positioned in an area of overlap between two header stations and wherein the mobile unit could potentially be serviced by either of the header stations.
(e) Readjustment of routing related information at header stations that are affected by the movement of a mobile unit.
An underlying concern in implementing these protocols is a consideration that mobile units are battery powered and that any implementation of the protocols must seek to conserve battery power. In this regard it is typically the case that an uplink transmission, from the mobile unit to the header station, places significantly more demand on battery power than the reception of a downlink transmission from the header station to the mobile unit. Thus, a protocol that minimizes the number of uplink transmissions is preferable to one that may require many uplink transmissions for accomplishing the same function.
The following U.S. Patents and articles are made of record for teaching various aspects of mobile communication.
The following two U.S. Patents show communication systems having overlapping coverage areas. U.S. Pat. No. 4,597,105, Jun. 24, 1986, entitled "Data Communications System having Overlapping Receiver coverage Zones" to Freeburg and U.S. Pat. No. 4,881,271, issued Nov. 14, 1989, entitled "Portable Wireless Communication Systems" to Yamauchi et al. Yamauchi et al. provide for a hand-off of a subscriber station from one header station to another by the header station continually monitoring the signal strength of the subscriber station.
The following U.S. patents teach various aspects of wireless communication networks.
In U.S. Pat. No. 4,792,946, issued Dec. 20, 1988, entitled "Wireless Local Area Network for Use in Neighborhoods" S. Mayo describes a local area network that includes transceiver stations serially coupled together in a loop.
In U.S. Pat. No. 4,777,633, issued Oct. 11, 1988, entitled "Base Station for Wireless Digital Telephone System" Fletcher et al. describe a base station that communicates with subscriber stations by employing a slotted communications protocol.
In U.S. Pat. No. 4,730,310, issued Mar. 8, 1988, entitled "Terrestrial Communications System" Acampora et al. describe a communications system that employs spot beams, TDMA and frequency reuse to provide communication between a header station and remote stations.
In U.S. Pat. No. 4,655,519, issued May 12, 1987, entitled "Wireless Computer Modem" Kirchner et al. disclose a wireless modem for transferring data in a computer local area network.
In U.S. Pat. No. 4,639,914, issued Jan. 27, 1987, entitled "Wireless PBX/LAN System with Optimum Combining" Winters discloses a wireless LAN system that employs adaptive signal processing to dynamically reassign a user from one channel to another.
In U.S. Pat. No. 4,837,858, issued Jun. 6, 1989, entitled "Subscriber Unit for a Trunked Voice/Data Communication System" Ablay et al. disclose a trunked voice/data subscriber that operates in either a voice mode or one of three data modes.
In U.S. Pat. No. 4,852,122, issued Jul. 25, 1989, entitled "Modem Suited for Wireless Communication Channel Use" Nelson et al. disclose a wireless communication system and, specifically, a modem that communicates digital data with data terminal equipment.
In U.S. Pat. No. 4,926,495, issued May 15, 1990 entitled "Computer Aided Dispatch System" Comroe et al. disclose a computer aided dispatch system that includes a master file node and a plurality of user nodes The master file node maintains a record for each subscriber and automatically transmits an updated record to each dispatcher attached to a subgroup in which the subscriber operates.
In U.S. Pat. No. 4,456,793, issued Jun. 26, 1984, W. E. Baker et al. describe a cordless telephone system having infrared wireless links between handsets and transponders. The transponders are wired to subsystem controllers which are in turn wired to a system controller. The central controller polls the cordless stations every 100 milliseconds to detect cordless station locations and to identify "missing" cordless stations.
In U.S. Pat. No. 4,807,222, issued Feb. 21, 1989 N. Amitay describes a LAN wherein users communicate with RF or IR signals with an assigned Regional Bus Interface Unit (RBIU). Protocols such as CSMA/CD and slotted ALOHA are employed in communicating with the RBIUs.
In commonly assigned U.S. Pat. No. 4,402,090, issued Aug. 30, 1983, F. Gfeller et al. describe an infrared communication system that operates between a plurality of satellite stations and a plurality of terminal stations. A host computer communicates with the terminal stations via a cluster controller and the satellite stations, which may be ceiling mounted. Communication with the terminal stations is not interrupted even during movement of the terminal stations.
In IBM Technical Disclosure Bulletin, Vol. 20, No. 7, December 1977 F. Closs et al. describe the use of both line-of-sight and diffuse transmission of infrared signals for wireless communications between a ceiling-based controller and a plurality of terminals.
In IBM Technical Disclosure Bulletin, Vol. 24, No. 8, page 4043, January 1982 F. Gfeller describes general control principles of an infrared wireless network incorporating multiple ceiling mounted transponders that couple a host/controller to multiple terminal stations. Access to the uplink channel is controlled by a Carrier Sense Multiple Access/Collision Detection (CSMA/CD) method.
What is not taught by this prior art, and what is thus an object of the invention to provide, are communication methodologies that realize, in a wireless communications network, the protocols (a)-(e) described above.
A further object of the invention is to accomplish the above referenced protocols (a)-(e) in a manner that makes efficient use of battery power of mobile units in a wireless communications network.